The present invention relates to the technical field of producing activated carbon.
The present invention relates more particularly to an apparatus and also a process for producing activated carbon, in particular by carbonization and subsequent activation of suitable polymeric starting materials, such as sulphonated polymers.
Activated carbon has highly non-specific adsorption properties and for this reason is the most widely used absorbent. Statutory requirements as well increasing environmental awareness are leading to an increasing demand for activated carbon.
Activated carbon is generally produced by carbonization and subsequent activation of suitable carbonaceous starting materials. Starting materials which lead to economically viable yields are generally preferred, since the weight losses caused by detachment of volatile constituents during carbonization and by burn-out during activation are appreciable. For further details concerning the production of activated carbon, see for example H. v. Kienle and E. Bäder, Aktiv-kohle und ihre industrielle Anwendung, Enke Verlag Stuttgart, 1980.
Carbonization, also known as pyrolysis, describes the conversion of the carbonaceous starting material into carbon. The process of carbonizing the aforementioned polymeric, in particular sulphonated, organic starting materials has the effect of detaching volatile constituents such as SO2 in particular to destroy the functional chemical groups, sulphonic acid groups in particular, to form free radicals which effect the pronounced crosslinking without which there would be no pyrolysis residue (=carbon).
Carbonization is followed by activation. The basic principle of activation consists in some of the carbon generated by carbonization being selectively and deliberately broken down under suitable conditions. This gives rise to a large number of pores, cracks and fissures, and the surface area per unit mass increases appreciably. Activation thus involves a deliberate burn-out of the previously carbonized material. Since carbon is broken down during activation, this operation leads to a loss of substance which is appreciable in some instances and which under optimum conditions equates to an increase in the porosity and an increase in the internal surface area and of the pore volume. Activation is therefore effected under selective/controlled, generally oxidizing, conditions.
The condition or constitution of the activated carbon produced—finely or coarsely porous, firm or brittle—is also dependent on the starting material. Customary starting materials are coconut shells, wood wastes, peat, bituminous coal, pitches, but also particular plastics, which play a certain part in the production of woven activated carbon fabrics inter alia.
Various forms of activated carbon are used: carbon powder, splint coal, granulocarbon, moulded carbon and also, since the end of the 1970s, activated carbon in spherical form (“spherocarbon”). Spherical activated carbon has a number of advantages over other forms of activated carbon such as carbon powder, splint coal, granulocarbon and the like, making it valuable or even indispensable for certain applications: it is free-flowing, hugely abrasion-resistant (i.e. dustless) and very hard. Owing to its high price, however, its use is essentially limited to protective suits and high-performance filters for noxiants in air streams.
Spherocarbon is in great demand on account of its specific shape, but also on account of its extremely high abrasion resistance for particular fields of use for example, examples being sheet filters for protective suits against chemical poisons and filters for low noxiant concentrations in large volumes of air. For instance, when reticulated, large-cell polyurethane foams are loaded with activated carbon as described in DE 38 13 563 A1, only a very free-flowing carbon can be used if optimal coverage of the inner layers of the foam material as well as the outer layers is to be achieved. The manufacture of protective suits against chemical poisons on the lines of DE 33 04 349 C3 for example can likewise utilize only a highly abrasion-resistant carbon, and only spherocarbon fits this description.
Spherocarbon is currently still being mostly produced by multistage processes which are very costly and inconvenient. The best-known process consists in spherules being produced from coal tar pitch and suitable asphaltic residues from the petrochemical industry and oxidized (to render them unmeltable), carbonized and activated. For example, spherocarbon can be produced from bitumen in a multistage process. These multistage processes are very cost-intensive and the associated high price of this spherocarbon prevents many applications wherein spherocarbon ought to be preferable by virtue of its properties.
There have consequently been various attempts to produce high-grade spherocarbon in some other way. It is prior art to produce spherocarbon by carbonization and subsequent activation of new or used ion exchangers containing sulphonic acid groups, or by carbonizing ion exchanger precursors in the presence of sulphuric acid and subsequent activation, wherein the sulphonic acid groups and the sulphuric acid respectively have the function of a crosslinker, and the yields obtained, which do not depend on whether ready-produced cation exchangers or unsulphonated ion exchanger precursors are used as starting materials, being about 30% to 50%, based on organic/polymeric starting material. Such processes are described for example in DE 43 28 219 A1 and in DE 43 04 026 A1 and also in DE 196 00 237 A1, including the German patent-of-addition application DE 196 25 069 A1. But these processes are disadvantageous and problematic particularly because of the large amounts of sulphur dioxide released (about 1 kg of SO2 per kg of end product) and also because of the (partly) associated corrosion problems in the manufacturing equipment. When used ion exchanger resins, in particular used cation exchanger resins, are used as starting materials, there is also the problem that these, although they have been washed with acid, are contaminated with cations which then accumulate in the end product, so that the production of major amounts of spherocarbon in consistent quality is consequently very difficult. When ion exchanger precursors, i.e. polymer spherules without exchanger groups like sulphonic acid groups, are used, it is additionally necessary to use large amounts of sulphuric acid and/or oleum for the crosslinking during the carbonization.
WO 98/07655 A1 describes a process for producing spherules of activated carbon wherein a mixture comprising a distillation residue from diisocyanate production and a carbonaceous processing assistant with or without one or more further added substances is processed into free-flowing spherules which are subsequently carbonized and then activated. This process likewise releases, in the course of the carbonizing step, large pulses of decomposition products, which is associated with the problems described above.
Commonly assigned WO 01/83368 A1 relates to an improved process for producing activated carbon wherein the requisite process steps of carbonization on the one hand and activation on the other are carried out separately from each other in that the carbonization is carried out as a continuous operation while the postcarbonization and activation is carried out as a batch operation. This process is mainly based on the separation of the corrosive phase (i.e. precarbonization, associated with SO2 emissions) from the high-temperature phase (activation). This is because precarbonized starting material is no longer corrosive; i.e. corrosive materials/gases are no longer formed when the temperature is raised any further.
Furthermore, the commonly assigned printed publications DE 2004 036 109 A1, DE 10 2005 036 607 A1 and also WO 2005/016819 A1 disclose apparatuses for producing activated carbon.
However, the processes and apparatuses for producing activated carbon which are known from the prior art are usually concerned with improving partial aspects only and do not provide a holistic approach which takes account of all problems arising in activated carbon production, particularly the high energy requirements, the use of cost-intensive starting materials and chemicals, the emission of offgases, the loss of energy in the individual processing stages, and the like.